At which pH is Alcian blue used to stain sulfated and carboxylated acid mucopolysaccharides and sialomucins?

• A. pH 2.5
• B. pH 7.0
• C. pH 1.0
• D. pH 5.0

Answer: (A)

The main concept is that Alcian blue staining depends on the ionization state of acidic groups in mucins. Alcian blue is a positively charged dye that binds to negatively charged sulfate and carboxyl groups on mucopolysaccharides. Whether these groups are negatively charged depends on the pH of the staining solution.

At pH 2.5, both sulfated and carboxylated groups are deprotonated and carry negative charges, so Alcian blue binds to sulfated mucopolysaccharides, carboxylated mucopolysaccharides, and sialomucins, producing a blue stain. This pH is chosen specifically to include all these acidic components.

If the pH were much lower, such as 1.0, the carboxyl groups would be largely protonated and lose their negative charge, so the stain would predominantly reflect sulfated mucins only and not the carboxylated ones or sialomucins. At neutral or higher pH values, the charge state would change in a way that reduces or alters binding, making the stain less specific for this broad class of acidic mucins.